Electronic noise generator



Sept. 27, 1949. E. B. NEWMAN ELECTRONIC NoIsE GENERATOR Filed Oct. 29,1945 l I I I I I I I I I I I I I I I I I l I I l I l l I I I I l I I I IPatented Sept. 27, 1949 #uN/Iran TAT-Es Parleur oFFlcE ELECTRONIC NOISEGENERATOR 'Edwin B. Newman, vCambridge Massi, assigner -t`o the UnitedStates of America as represented by the Executive 'Secretary 'of thelO'flce fof v'Scientific Research and Development Application octoberv29, 1945, "Serial Na 6255163' `6 Claims. (Cl. 25W-36') VThe invention'described herein may be manufactured and used by or for the Governmentfor governmental purposes without payment to me of any royalty thereon.

This hinvention relates to -noise generators and more especially to'electronic generation of airplane noise.

"The noise which vis encountered -in aircraft has twopfinci'palcomponents. The yiirst is that of the propeller which sendsout radially compression Waves of very vhigh intensity, producing acharacteristic'throbbingior humming. This noise canberepresented as adiscontinuousor line spectruir'i, vconsisting -of ya 'fundamental`frequency equal 'to the frequency with which the propeller tips passany given point, "and a series of higher frequencies which are'harmonics or multiples of the fundamental frequencies. The secondsourceoi noise turbulence inthe air stream, and such turbulence is to -befound associated with various irregularities presented by' an air shipto the smooth Tl'o'tv passing its surface. This noise consists 'of acontinuous or random spectrum. In other Words, A-this noise Ycontainsall frequencies from the lowest to `the highest audible sounds. Therearealso l'other sources of noise which may bedisre'gafrd'ed for thepurposes 'of this invention.

It 1'is an object of the invention to provide 1an apparatus `for theelectronic generation of noise, and to synthesize 4el'eetrical'ly adiscontinuous vor line spectrum, and a continuous or random spectruth.n'oth'e'r 'ob-'ject of the yin'vei'ition is toisolate adesired bandldf`frequencies fr'omfone of the spetia an'd'to mik the'-des`relf bar-1d offrequencies with "frequencies in the other of thelspectra, to'produceasnearly as possible the true 'principal coniphrl'itslo "lplanelnoisesAnother object of the invention 'is to modify electrical :devices usedih'pibauln'g "a ll'hefpectiiinahd a ladm lSpectrum in a manner intendedto facilitate the application of these devices to generation ofairp'iitle 'II'SGL the accompanying drawings:

Fig. 2 is a wiring diagram of noise generating apparatus.

There are two fundamental devices for electronically vr'roducing noises.The rst of these is the relaxation oscillator. The oscillator consistsof La condenser Awhich is charged at a gradually decreasing 'ratethrough ahigh resistance. When the poten'ti'al'across this condenserreaches a critical high value, Lthe gas 'containedfi'n theftube isionized and the condenser is rapidlydischarg'ed.

When the' potentialihasfallen to a-'second "critical low valuethecurrent 'throughthe tube 4no longer maintains ionizationand the tubeceases to conduct. A harmonic analysis of this'wave form reveal'stheffact that it contains a fundamental frequency corresponding with pericd:and a long series Iof higher harmonics. This fundamental circuit isused to provide line spectrum 'generators.

'There are vseveral methods yof generating the random noise spectrum,depending -upon the behav-iorfof electrons or of ions. 'The `most usefulsou-roe of random potentials' is 'a igasefilled diode. In 'this-case,current Tis carried `*principally by the krelatively largeandsloivm'oving ions of gas. Be-

cause' of their 'propertiesrthe momentary uetuation'sin the'cathode-toanode current of such a gas diode "tl 'Bftied thS'e plfducedby other sources. 1t iis to loe'.anticipatedthat the maximum output Willbepr-'odueed W'liensuch va ytube isop-erating near thecriticallyiomzation point. VInapplying such potentials top'urposes ofthis invention, it is necessary first to isolate and amplify this noise,andthen to pass it through such ltering networks as will give 'theproper distribution of 'energy Vin the various regions of the soundspectrum.

In accordance with the present invention, the two types of soundgenerating equipment, i. e., relaxation oscillator and the -gas-lleddiode, are combined on one chassis, as illustrated in the blockdiagram-of Fig, Y1,an'd the wiring diagram of Fig. 2.

Considering 'the Wiring diagram more in detail, the 2051 tube is thegenerator of the continuous spectrum. The output of this tube -is fedthrough a tube GSJ'I to the No. 1 g'ridof a 1.612 mixer -amplier tube.At the same time the line spectrum is Vgenera-'ted in the 8'84 tube, theoutput of vhich is fed to the No. `3 grid of Ythe 1612 mixer ube.

The output 'of the mixer tube -is lead to -a second 6ST? tube, triode'connected, in the grid circuit oi which is connected thefrequency-control network. The latter network is made up of resistorsR-13, R-14, andthe condenser C-18. The power supply u sesa 5Zk4'tube,the output -of which is stabilized with two VR--30 voltage-regulatortubes connected in series.

The amplifier fand power supply components of this unit may -b'e modiedWithout substantially `affecting the operation Lvof the noise generator.Some Valueslmay loe 'changed without changing the operation of v1thedevice. For insta-nce, the 25henry choke coil in the plate lead tothe 884 tube should be fairly close to this rated value. In the sameway, the values of the three condensers, C-13, C-16, and C-17, and theresistors R-20 and R-24, in the network associated with this tube,should not be changed. Other values which should be adhered to fairlyclosely are the resistors R-13 and R- 14 and the condenser C-18 whichform the frequency control network in the grid circuit of the last tube.R-25 may normally be omitted. However, it may be included in case it isdesired to increase the high frequency output of the generator. Changesin other circuit values should be consistent with good engineeringpractice.

In making up the circuit for the line spectrum generator, it has beenfound advantageous to utilize current flowing into and out of thecondenser C-13 for generating the desired potential. This purpose hasbeen accomplished by inserting a resistor R-ZO in the lead from thecondenser to the ground, and taking olf the potential across thisresistor. 1t has been found that more of the energy from such agenerator will be found in the lower frequencies if the rate ofdischarge ."s slowed down somewhat. For this purpose, a large inductanceLi, as shown in Fig. 2, has been inserted in the lead of the gasdischarge tube. It is desirable to stabilize the frequency of the linespectrum generator, and this object is accomplished by using a triode884 in place of a diode and applying a small alternating currentpotential to the grid through R-5. It has also been found essential toprovide a band of frequencies selected from the lower harmonics of theoutput of the line spectrum generator. This important objective has beenattained by passing the generator output through filters as shown inFig. 2, which give a characteristic band made up of components between125 cycles and 250 cycles. One would consist of C17 and R-24; the otherwould be made up of the sloping network consisting of R13, R14, and Cia.

The following list of components indicate values that have been found toprovide the desirable noise reproduction although these values may bechanged to alter the resultant noise without departing from the spiritand scope of this invention.

RlRcRaR-Rs 50,000 ohms 1w RsR'zRaRe 100,000 ohms 1w RioRii 1,000 ohms 1wR12R13Ri4 500,000 Ohms 1/gw R l megohm 1w RmRnRis 250,000 ohms 1w R19 2megohm 1w R20 10,000 ohms 1w R21R22 5,000 Ohms 1w 12.2312241325 0.5megohm potentiometer R25 2,000 ohms 10a R27 4,000 ohms 10e adjustable L125 henry choke T1 Power transformer S1. Swith C1C2C3C4C5C6 40 afd.,electrolytic, 450 V.

working C'ICBCQ 0.1 Iafd., paper CioCiiClz 20 afd., electrolytic, 25 V.

working C13 .01 afd., paper C14C1s .25 afd., paper C16 .05 afd., paperC17 .003 afd., paper C18 .005 afd., paper G19 .5 Aafd., paper The mostimportant features of this circuit arrangement are found in connectionwith the inductance element used with the line spectrum generator, andthe filter elements used to produce a selective band of frequencies ofthe line spectrum generator. The inductance element is particularlyeffective in generating low frequency components of airplane noise whichare sometimes difficult to produce in proper values. The frequencycontrol network is capable of emphasizing a particular group ofcomponents which have been found to closely simulate similar componentsoccurring in average airplane noises. This particular band offrequencies from the line spectrum generator, as already disclosed, hasbeen found to occur between 125 and 250 cycles. Determination of thisband of frequencies is an important element in the method of modifyingthe line spectrum to simulate a similar component in airplane noise. Afurther point is to modify the output of the random generator so as todecrease the intensity of frequencies above 250 cycles, at the rate of afrequency range of 5-7 decibels per octave.

Any good power amplifier with a Wide range of frequency response and areasonably small distortion will adequately meet the needs of this noisegenerator. It should have good dynamic power handling capacity. Thepower needed will depend principally upon the sound level to beachieved, and upon the characteristics of the room in which the noise isto be produced. For example, a battery of six 50-watt power amplifiersis sufficient to produce an overall noise level of better than db in aroom of 10,000 cu. ft. volume, with hard plaster walls. For mostpurposes, commercially available 50-watt power amplifiers are quitesuitable.

If more than 50-watt power is required, these units can be used inparallel with suitable adjustment of the output circuit to match theimpedance of the speakers used. Loudspeaking equipment of the typeparticularly utilized in systems for theatre application have been foundto give best results. Current engineering practice indicates thedesirability of using a dividing network and both a low frequency and ahigh frequency horn. Satisfactory results have been obtained from asystem which includes a Jensen PLJ18B in a brass reflex cabinet tohandle frequencies up to 400 cycles. This apparatus was set up in abrick-walled room of 2500 cu. ft. volume.

The noise generating equipment described is effective in synthesizing anaverage noise generally corresponding to a noise field in which thepilot of a plane is required to operate. By simulating a noise field bymeans of electronic generation, more realistic training conditions areavailable to student pilots and others. This is especially useful indetermining the ability of the pilot to hear and understand directionsreceived in the earphones or other types of communication equipment.Electronically generated noise can also be made use of in various othertraining situations, covering a variety of types of equipment.

Having thus described my invention, what I claim is:

l. A device for electrically simulating the noise of an airplane inflight over a sound producing means comprising a random spectrumgenerator for producing oscillations corresponding to the oscillationsof noise produced by air turbulence about the airplanes structure in theairstream, a line spectrum generator having an inductance means in itsoutput circuit effective in generating low frequency components ofairplane noise wherein said line spectrum generator produces afundamental frequency and harmonics thereof corresponding to thefrequencies produced by noise from the propeller, means to combine theoutput oscillations of both spectrum generators in mixer amplifiercircuit, means coupling the mixer amplifier output to a network havingan adjustable resistance included therein for adjusting the frequencytransmission characteristic, and an output power amplifier coupled tosaid network whereby the mixed oscillations are amplified for soundproduction.

2. An electron device for simulating the noises produced by an airplanein flight comprising a relaxation oscillator in a circuit having aninductance in its output effective in generating low frequency-components of airplane noise wherein said relaxation cscillatorproduces voltages of a fundamental frequency and harmonics thereofcorresponding in frequencies to those of propeller noise, a gas-filledtube for producing voltages of random frequencies corresponding tofrequencies produced by the noise of air turbulence of the airplanestructure in the air stream, means connecting the output voltages ofsaid relaxation oscillator and said gas-lled tube to a mixer amplifiertube, a network having a fixed resistance coupled to a second fixedresistance arranged in parallel with a variable resistance and acondenser wherein the Variable resistance is adjustable to control thefrequency transmission characteristics to emphasize particular frequencycomponents closely simulating frequency components of noise ofairplanes, means coupling the output of said mixer amplifier tube withsaid network, and means for connecting said network with a widefrequency range sound producing system for converting the mixedoscillations of said relaxation oscillator and said gas-filled tube intosound of proper tone and volume to simulate airplane noises in iiight.

3. An electronic device for generating a plurality of audio frequenciescorresponding to the audio frequencies in the noise produced by aircraftin flight comprising, a relaxation oscillator circuit having aninductance means in its output effective in slowing down the rate ofdischarge to generate low frequency components of airplane noise whereinsaid relaxation oscillator produces voltages of a fundamental frequencyand harmonics thereof corresponding to frequencies of propeller noise, agas discharge tube circuit for producing voltages of random frequenciescorresponding to frequencies of noise produced by air turbulence of theairplane structure in the air stream, the output voltages of saidrelaxation oscillator circuit and the output Voltages of said gasdischarge tube circuit being connected to separate control grids of amixer ampliner tube, means connecting the output of said mixer amplifiertube to a network having a fixed resistance coupled in series to asecond Xed resistance arranged in parallel with a variable resistanceand a condenser wherein the variable resistance is adjustable to controlthe frequency transmission characteristic to emphasize particularfrequency components closely simulating frequency components of noise ofairplanes, means connecting the output of said network through apre-amplifier means to a power amplifier means, and means for connectingsaid power amplifier to a wide frequency range sound producing systemwhereby the propeller and air turbulence noises produced by an airplanein flight may be simulated in tone and sound volume.

4. An electronic device as set forth in claim 3 wherein the output ofthe relaxation oscillator circuit includes lters to give acharacteristic frequency band made up of components between cycles and250 cycles,

5. An apparatus as set out in claim 4 in which said relaxationoscillator circuit comprises a condenser, means providing a chargingcircuit for said condenser, a resistance in said charging circuit acrosswhich said fundamental harmcnic frequencies are produced, meansproviding a discharge circuit for said condenser comprising a gas-filledtube acting to discharge said condenser when the condenser voltagereaches a predetermined value, and inductance means in series with theanode of said gas-filled tube for modifying the discharge characteristicof said v condenser.

6. An apparatus as set forth in claim 5 wherein f the gas discharge tubeof said gas discharge tube circuit is connected in series with aresistance across a source of direct Voltage whereby said randomfrequency voltages are developed across said resistance.

EDWIN B. NEWMAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,994,902 Trouant Mar. 19, 19352,097,066 Hoover Oct. 26, 1937 2,121,142 Dudley June 21, 1938 2,139,023Kock Dec. 6, 1938 2,165,509 Ring et al July l1, 1939 2,183,248 RiesZDec. 12, 1939 2,194,298 Dudley Mar. 19, 1940 2,207,620 Hilferty Juiy 9,1940 2,253,975 Guanella Aug. 26, 1941 2,353,499 Furington July 1l, 19442,354,699 Owens Aug. 1, 1944 2,373,560 Hanert Apr. 10, 1945 2,416,307Grieg Feb. 25, 1947 2,416,327 Labin Feb. 25, 1947 2,445,233 MontgomeryJuly 13, 1948

